International Conference on Gas Hydrates (ICGH) (6th : 2008)

ANALYSIS ON CHARACTERISTICS OF DRILLING FLUIDS INVADING INTO GAS HYDRATES-BEARING FORMATION Ning, Fulong; Jiang, Guosheng; Zhang, Ling; Bin, Dou; Xiang, Wu 2008-07-31

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   ANALYSIS ON CHARACTERISTICS OF DRILLING FLUIDS INVADING INTO GAS HYDRATES-BEARING FORMATION    Fulong Ning ????    Guosheng Jiang     Ling Zhang    Dou Bin    Wu Xiang Faculty of Engineering China University of Geosciences 388 Lumo Road, Wuhan, Hubei, 430074 China   ABSTRACT Formations  containing  gas hydrates  are  encountered both during  ocean drilling  for  oil  or  gas,  as well  as  gas  hydrate  exploration and exploitation. Because  the  formations  are  usually  permeable porous  media,  inevitably  there  are  energy  and mass  exchanges  between  the  water-based drilling fluids  and  gas  hydrates-bearing  formation  during  drilling,  which  will  affect  the  borehole?s stability and safety. The energy exchange is mainly heat transfer and gas hydrate dissociation as result  of  it.  The  gas  hydrates  around  the  borehole  will  be  heated  to  decomposition  when  the drilling fluids? temperature is higher than the gas hydrates-bearing formation in situ. while mass exchange  is  mainly  displacement  invasion.  In  conditions  of  close-balanced  or  over-balanced drilling,  the  interaction between drilling  fluids  and hydrate-bearing  formation mainly  embodies the invasion of drilling fluids induced by pressure difference and hydrate dissociation induced by heat  conduction  resulting  from  differential  temperatures.  Actually  the  invasion  process  is  a coupling  process  of  hydrate  dissociation, heat  conduction  and  fluid displacement.  They  interact with  each  other  and  influence  the  parameters  of  formation  surrounding  the  borehole  such  as intrinsic mechanics, pore pressure, capillary pressure, water and gas saturation, wave velocity and resistivity.  Therefore,  the  characteristics  of  the  drilling  fluids  invading  into the  hydrate-bearing formation  and its  influence  rule  should be  thoroughly  understood when  analyzing  on wellbore stability, well logging response and formation damage evaluation of hydrate-bearing formation. It can be  realized by  establishing  numerical  model  of  invasion coupled with hydrate  dissociation. On the  assumption that  hydrate  is  a  portion of  pore  fluids  and its  dissociation is  a  continuous water  and gas  source  with no uniform  strength, a  basic  mathematical  model  is  built  and  can  be used to describe  the  dynamic  process  of  drilling  fluids  invasion by  coupling  Kamath?s  kinetic equation of heated hydrate dissociation into mass conservation equations.   Keywords: gas hydrates-bearing formation, drilling fluids, hydrate dissociation, invasion, model                                                         ?  Corresponding author: Ning Fulong, E-mail: nflzx@cug.edu.cn. This work was supported by NSFC(No.50704028), 863 Program (No.2006AA09Z316), Program for New Century Excellent Talents in University (No.NCET-05-0663) and the Research Foundation for Outstanding Young Teachers, China University of Geosciences (Wuhan)(No.CUGQNL0623, CUGQNL0725) NOMENCLATURE As     total surface area of hydrates in a unit volume  k      absolute  permeability  coefficient  of  porous medium[m2]. Kd?  dissociation  constant  of  hydrate  under constant-pressure heated  krw    relative permeability coefficient of water  krg      relative permeability coefficient of gas  Mg      methane molecular weight Mw     water molecular weight,  mg          mass  rate  of  gas  which is decomposed from hydrate[kg/s] Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  nw      amount  of  water  molecules  in  hydrate molecule  Pw         flow pressure of water[MPa] Pg         flow pressure of water and gas[MPa] P           pore pressure Sh          hydrate saturation Sw         water saturation Sg          methane saturation T           temperature of unit volume Teq          phase  equilibrium  temperature  of  hydrate under P  Greek letters ?w          water density[kg/m3] ?g           gas density[kg/m3]  ?h           hydrate density[kg/m3] a0w         kinetic viscosity of water[Paa1 s] a0g          kinetic viscosity of gas[Paa1 s]  Subscripts d            dissociation eq          equilibrium g            gas h            hydrate w           water s             surface  INTRODUCTION Gas  hydrates  are  regarded  as  the  potentially replaceable  energy  in  the  future.  According  to Klauda  and Sandler[1],  there  is  74 400Gt  of  CH4 trapped in hydrates  buried  in oceanic  zone,  which is  3  orders  of  magnitude  larger  than  worldwide conventional  natural  gas  reserves.  And  in order  to explore  and  exploit  the  gas  hydrate  stored  in  the deep underground, drilling is inevitably needed. During  the  drilling  process,  drilling  fluids  (water-based  drilling  fluids  are  mentioned  in  this  paper) inevitably  contact  with  the  gas  hydrate-bearing formation  and  exchange  energy  and  substances with  it.  Just  as  drilling  activities  prove  that  the interactions  between  drilling  fluids  and  formation, such  as  seepage,  diffusion,  and  heat  transfer,  can change  the  stress  status  of  rocks  surrounding borehole.  Especially  during  drilling  in  marine hydrate-bearing  formation,  the  formation  is  a porous  medium  with  comparatively  high permeability  and  temperature  and  pressure  in  the borehole  is  changing  during  drilling,  which  will break  the  equilibrium  condition  of  hydrate?s thermodynamics and rocks mechanics in-situ. As a result,  it  will  lead  to  hydrates  dissociation  and affect  the  wellbore  stability  and drilling  safety[2]. Collett  et  al[3]and  Maurer[4]  once  had  detailed discussions  about  the  issues  of  wellbore  stability and  drilling  safety  involved  in  hydrate  drilling. Sheila et al[5], Tan et al[6] and Freij-Ayoub[7] had a further discussion of how to establish a model of hydrate  wellbore  stability.  Therefore,  the  interact-tions between drilling fluids and formations during the  drilling  process  in hydrates-bearing  formations can  not  be  neglected.  The  researches  of  wellbore stability  and  borehole  safety  are  all  based  on  the analysis  and  understanding  of  interaction  of  heat and  fluids  between  drilling  fluids  and  hydrate-bearing formation.   ANALYSIS  ON  INTERACTIONS  BETWEEN DRILLING  FLUIDS  AND  HYDRATE-BEARING FORMATION The  interactions  between  drilling  fluids  and hydrate-bearing  formation  is  more  complicated than that  of  the  ordinary  oil  or  gas  formation,  and its representations can be concluded into following aspects.  Mass exchanges Considering  environmental  protection  and operating cost, the most widely used mud is water-based  drilling  fluids  system  in the  oceanic  drilling of  oil  and gas.  In the  same  way,  the  hydrate  also exists  in/under  seafloor  sediments  of  deep  oceans, and  the  water-based  drilling  fluids  have  a  good inhibitive  ability  towards  hydrate[8].Therefore, the popularly  used  system  in  the  drilling  for exploration and exploitation of  oceanic  hydrate  is the  water-based  drilling  fluids.  The  higher  the density  of  drilling  fluids  is,  the  higher  the  bottom hole  pressure  will  be.  When the  pressure  is  higher than  pore  pressure,  the  drilling  fluids  will  flow through  the  filter  cake  and  permeate  into  the formations surrounding the well, thus will displace the original pore fluids and affect the pore pressure and  permeability  of  formation.  Contrariwise,  the bottom  hole  pressure  will  be  lower.  When  it  is lower  than  the  formation  pressure,  the  fluids surrounding  the  well  will  reversely  flow  into  the well,  and  this  is  not  good  for  the  balance  of borehole mechanics. At the same time, hydrate will decompose  under  depressurization, it  is  worse  for the  wellbore  stability  and  borehole  safety. Therefore,  to  keep  the  pressure  in borehole  higher than  the  pore  pressure  (but  not  higher  than fracturing pressure) is an advisable safe manner for hydrate  drilling.  Besides,  the  mineralization  of drilling  fluids  is  different  from  that  of  the symbiosis  water  in  formation.  A  large  amount  of salt  is always  added into the  drilling  fluids for  gas hydrate  as  inhibitors  and  the  dissociation  of hydrates  will  also  decrease  the  mineralization degree  of  the  pore  water  in  formation,  both  will cause that  the  mineralization degree  of the  drilling fluids  is  higher  than  that  of  the  pore  water  in hydrate-bearing formation, therefore, the difference of chemical potential will drive pore water towards hole which is opposite to the hydraulic differential pressures  exists  between  hole  pressure  and formation.  But  under  the  condition  of  over-pressure  drilling,  filter  cake  can  be  regarded  as  a semi-permeable  membrane,  water  as  well  as positive  and  negative  ions  penetrate  into  the formation through this  layer,  and it  is  independent of  mineralization degree  of  pore  water.  So,  under the  aforementioned  condition,  one  of  interactions between  the  drilling  fluids  and  hydrate-bearing formation around the borehole is represented as the seepage  and  displacement  under  hydraulic differential pressure.   Dissociation of hydrate surrounding borehole At  the  moment  of  opening  the  borehole,  the pressure  release  and  friction  heat  inside  the  well will  cause  inevitable  dissociation  of  the  hydrate surrounding  the  well.  Besides,  hydrates  are  buried under  shallow  marine  sediments,  the  formation temperature  is  low.  So  the  temperature  of  drilling fluids  is  often  higher  than  the  formation temperature in-situ. After the circulation of drilling fluids is established, the hydrate-bearing formation will  be  heated  and  thus  the  hydrate  dissociation will  be  speeded  up.  Because  the  mechanical properties  of  the  hydrate-bearing  formation  are quite different from those of the formation without hydrate[9-11],  the  hydrate  dissocia-  tion  will influence  the  stress  distribution  of  rock surrounding  borehole[7].  What?s  more,  it  will  add water  and gas  in the  bore,  causing  the  increase  of pore  pressure[7][12],  which  also  influence  the effective  stress  distribution  of  rock  surrounding borehole.  Therefore,  the  dissociation  of  hydrate surrounding  well  caused  by  the  heat-transfer between  the  drilling  fluids  and  hydrate-bearing formation  is  another  main  representation  of interactions  between  them,  and  it  is  also  the  key factor  to  affect  the  wellbore  stability  and  drilling safety of the hydrate-bearing formation.  Heat exchange The  change  of  hole  temperature  makes  hydrate unable to keep stability, and it also causes borehole to  generate  additional  heat  stress,  which  will influence the  mechanical  stability  of  borehole.  But then,  because  the  storage  location  of  marine hydrate is over  300m  water  depth, the  temperature of  drilling  fluids  circulation is  not  high. While  the hydrate  formation often lies  under  seafloor  within 0-800m  distance,  so  formation  temperature  is  not high  either.  Owing  to  both  of  the  aspects,  the difference  in  temperature  between  drilling  fluids and  formation  is  not  large,  and  the  heat  stress caused by  heat  exchange  is  not  as  obvious  as  that in  the  high-temperature  deep  well  and  can  be neglected.  The  heat  exchange  mainly  influences the hydrate stability near borehole. Thereby,  the  interactions  between  drilling  fluids and  hydrate-bearing  formation  during  drilling process  are  mainly  represented  as  drilling  fluids invading  into  the  hydrate-bearing  formation through  seepage  and  displacement  under differential pressures and the heated dissociation of hydrate  by  heat  transfer  under  difference  in temperature (Fig.1). These two is coupled together. Both  the  invasion  of  drilling  fluids  and  hydrate dissociation  will  increase  the  pore  pressure  and decrease the effective stress. At the same time, they also  change  the  formation  permeability,  acoustics parameters  and  resistance  rate.  Therefore,  the research  on  characteristics  of  drilling  fluids invading  into  hydrates-bearing  formation  is  the basis  for  future  analysis  of  wellbore  stability  as well  as  evaluation  of  well  logging  response  and formation damage.  ANALYSIS  ON  CHARACTERISTICS  OF DRILLING  FLUIDS  INVADING  INTO HYDRATES-BEARING FORMATION   Invasion process of drilling fluids Drilling  fluids  will  seep  into  the  hydrate-bearing formation  and  displace  the  water  and  gas  in  the formation under differential pressures. At the same time,  theirs  higher  temperature  cause  dissociation of  hydrate  surrounding  borehole.  The  decompose water and gas as well as the drilling fluids invasion will flow deep into formation driven by the newly-invaded  drilling  fluids.  Therefore,  the  invasion  of drilling  fluids  in  the  hydrate-bearing  formation  is coupled with hydrate dissociation and heat transfer.   Fig.1 Interactions between drilling fluids and hydrate-bearing formation   When the  borehole  is  opened, the  solid-phase  and liquid-phase  in  drilling  fluids  immediately penetrate into the borehole, which causes the water content  surrounding  the  borehole  to  increase largely.  At  the  same  time,  the  hydrate  surrounding borehole is quickly decomposed into water and gas because  of  the  rapid  change  of  temperature  and pressure,  thus  the  water  content  surrounding  the borehole  is  further  increased.  If  the  formation medium  is  sandstone,  it  will  possibly  be  liquefied. The  dissociation  of  hydrate  improves  the penetrability  and  speeds  up  the  invasion  speed, which makes against forming filter cake. Therefore, during  the  period from  the  opening  of  borehole  to the  completed dissociation  of  hydrate  surrounding the  borehole,  the  borehole  is  the  most  unstable. With the drilling going on, the filter cake is formed gradually,  and  the  mud  filtrate  seeps  through  the filter  cake  and  further  invades  into  formation.  At this  stage,  the  temperature  change  is  not  dramatic, thus  the  speed  of  hydrate  dissociation  is comparatively  slow.  In  addition, the  invasion  and hydrate  dissociation increase  pore  pressure,  which also  slows  the  hydrate  dissociation.  During  the later  phase  of  invasion,  gas  and  water  content increase  and pore  pressure  continuously  rises,  the gas  is  compressed,  and  mud  filtrate  almost  stops seeping,  but  diffusion still  exists.  At  this  stage,  if the  temperature  and  pressure  inside  pores  are appropriate,  part  of  the  gas  and water  will  reform hydrate again. Therefore  after  the  invasion  becomes  stable,  the invasion  section  surrounding  the  borehole  can  be divided  into  five  layers.  As  Fig.  2  illustrates,  the first  layer  contains  filter  cake,  all  the  components of  drilling  fluids  touch  directly  with  it,  and  the thickness  of  this  layer  may  range  from  several millimeters  to  several  centimeters.  It  does  not contain hydrate  or  free  gas  but  muds.  The  second layer does not contain hydrate and free gas too, but contains mud filtrate. In the third layer, there exists mud  filtrate  and  free  gas  and  no  hydrate.  In  the third  layer,  water  (including  water  in  the  filtrate, water dissociated from hydrate and connate water), gas (gas dissociated from hydrate and connate gas) and hydrate  are  three-phase  coexistence,  and  they are  in  dynamic  equilibrium.  The  last  layer  is  the hydrate-bearing  formation which keeps  its  natural characteristics  during  the  whole  drilling  process, and it does not interact with mud filtrate as well as the gas and water decomposed from hydrate at all. So the  invasion of  drilling  fluids  can be  described as  an issue  of  hydrate  boundary  movement  during dissociation, its  essence  is  an equilibrium  issue  of dynamics and thermodynamics. Only the existence of  seepage  and  dispacement  makes  this  issue become comparatively complicated.   Filtrate Filter cake Water, free gas layer In situ layer Water-gas-hydrate coexistence layer a2                  a3           a4          a5  a6Fig.2 Invasion layers of drill fluids Analysis  on  heat  transfer  during  the  drilling fluids invasion The invasion of drilling fluids into the oil and gas-based  formation  used  to  be  regarded  as  an isothermal  seepage  process,  and  the  influence caused  by  temperature  is  always  neglected. Actually, drilling process is a nonadiabatic process. The  circulation  of  drilling  fluids  and  invasion  of filtrate  into  borehole  formation  are  always accompanied  with heat  transfer.  Especially  during the  drilling  of  hydrate-bearing  formation,  because of  the  combined  actions  of  driller  friction, comparatively  high  circulation  temperature  of drilling  fluids  and  short  distance  between  storage location of marine hydrate and seafloor(the ground temperature  is  comparatively  low),  heat  will transfer  from  drilling  fluids  to  hydrate-bearing formation  through  framework  conduction  and convective  heat  transfer  of  invading  fluids,  which causes  formation  temperature  to  rise  and  the hydrate  surrounding  borehole  to  be  decomposed. Under  the  condition  of  over-pressure  drilling, temperature becomes a main impetus to the hydrate dissociation. Therefore in analyzing the invasion of drilling  liquid  into  hydrate-bearing  formation  and its  influence  to  wellbore  stability,  it  can  not  be regarded  as  an  isothermal  process.  Furthermore, hydrate  dissociation  itself  is  an  endothermic reaction  during  the  decomposing  process.  The formation  temperature  will  be  influenced,  and the change  of  temperature  will  influence  the  speed of hydrate dissociation at the same time. Therefore, in analyzing  the  characteristics  of  the  invasion  of drilling  fluids  into  hydrate-bearing  formation, temperature  should be  considered as  an important element, and it is a non isothermal process.  Mathematical  model  of  drilling  fluids  invasion coupled with hydrate dissociation According  to  the  aforementioned  analysis  on invasion  process  of  drilling  fluids  and characteris-tics  of  heat  transfer,  the  main  feature  of  the invasion  into  hydrate-bearing  formation  is  that temperature  change  and  hydrate  dissociation  are accompanied with the  invasion process.  Under  the condition  of  over-pressure  drilling,  temperature  is the  main  factor  in  hydrate  dissociation,  and  it controls  the  speed  and  range  of  the  dissociation. The  invasion  of  drilling  fluids  and  the  hydrate dissociation  change  the  pore  pressure,  and  thus influence  the  flow  of  pore  fluids  and  speed  of hydrate  dissociation.  Therefore,  the  invasion process  is  actually  a  coupling  process  of  hydrate dissociation,  heat  conduction  and  fluid  seepage. They      interact  with  each other  and influence  the parameters  of  formation surrounding  the  borehole such  as  intrinsic  mechanics,  pore  pressure, capillary  pressure,  water  and gas  saturation, wave velocity  and  resistivity.  Therefore,  building  an adequate  invasion  numerical  model  becomes  an effective approach to evaluate these influences and analyze  log  data,  wellbore  stability  and  formation damage evaluation. Holder  et  al.[13]built  a  mathematic  model  of hydrate  dissociation  under  depressurization,  the model  is  based on the  transfer  of  heat and mass in formation,  and  they  thought  the  sensitive  heat transfer  in  formation  provided  energy  for  the hydrate  dissociation.  Makogon[14]  supposed  that the  depressurized  dissociation  process  of  gas hydrate is similar as the solid melting. He used the classical  Stefan  problem  to  describe  the  process, and  built  a  basic  lineal  equation  to  describe  the movement  of  natural  gas  in  porous  medium  and heat transfer, but his model neglected the influence caused  by  the  water  generated  from  hydrate dissociation.  The  model  of  Goel  et  al.[15]  also neglected the  influence  on  the  gas  flow  caused by the water flow in the reservoir. Actually, the water will  block  the  hydrate  dissociation.  A  one dimensional  model  of  Yousif  et  al.[16]  considered the  movement  of  water  phase.  As  a  result,  they found  the  maximal  water  content  will  occur  in formation  during  the  hydrate  dissociation process, it  reduced  the  relative  permeability  of  gas  phase, influenced  gas  flow  and  thus  increased  pore pressure,  and  further  conversely  influenced  the hydrate  dissociation.  Moridis  et  al.(2002,2003) simulated  the  release,  phase  behavior  and nonisothermal  flow  of  methane  in  deep  sea  and permafrost area with universal numerical simulator TOUGH2 and program  module  HYDRATE.  Their mathematical  model  was  realized  through  resolv-ing  the  coupled  mass  and  thermal  conservation equations.  All  of  the  aforementioned  scholars considered  the  coexistence  area  of  gas-liquid-hydrate  as  a  border to separate  gas-liquid area  and hydrate-liquid  area,  and  regarded  hydrate dissociation as a quasi-static system.  In  fact,  from  the  beginning  of  the  forming  of  the hydrate-bearing  formation,  the  gas-liquid-hydrate three  phases  coexist  in  it.  The  invasion  of  the drilling fluids can be regarded as a reverse-process of  depressurization  exploitation,  but  because  the invasion  of  drilling  fluids  and  the  dissociation  of hydrate dynamically happen at the same time, they don?t  last  for  a  long  time,  and  the  hydrate dissociation itself will influence fluids seepage and heat  transfer.  Therefore,  the  aforementioned models  can  not  accurately  describe  the  dynamic process of invasion, and they can not make sure the changes  of  water,  gas  and  hydrate  saturation  and permeable  characteristics  of  a  certain  area  during the  invasion  process.  So,  the  kinetic  behavior  of hydrate  dissociation should be  considered into the mass  equations  and  build  a  new  mathematical model.  Without  regard to diffusion effect,  the  invasion of drilling fluids in the hydrate-bearing formation can be  described  as  a  seeping  displacement  and  heat transfer  of  multiphase  fluids  with  phase  change (hydrate dissociation) in porous medium. Obvious-ly, the influence of hydrate dissociation on seepage is  the  key  point  in  model  building.  In  this  way, hydrate  dissociation  can  be  treated as  a  portion of pore  fluids  and  a  continuous  water  source  and  gas source  with  no  uniform  strength.  Kamath?s  et  al. [17-18]  kinetic  equation  of  heated  hydrate dissociation  as  well  as  Brown?s  thermodynamics equation of hydrate phase balance are coupled into mass  conservation  equations,  and  a  thermal-flow-hydrate coupled theoretical model is established by considering  the  invasion  of  drilling  fluids  and hydrate  dissociation  in  hydrate-bearing  formation, its basic equations is like as following:  ( )gwwwww MMP?kk?St +?????? = ??     a71a8  ( ) ggggg mP?kk?St += ][g??                   a72a8  ( ) )gwwggh MMnMmSt +?= ??                 a73a8  T? C?kkP? C?kkMmtTeggwwwggh2)???????????????? +=+??a9a74a8  Where  Sh  is  hydrate  saturation,  Sw  is  water saturation,  Sg  is  methane  saturation.  ?w  is  water density,  kg/m3;  ?g  is  gas  density,  kg/m3,  ?h  is hydrate  density,  kg/m3.  mg  is  the  mass  rate  of  gas which is decomposed from hydrate, kg/s, Mg is the methane  molecular  weight,  Mw  is  water  molecular weight,  nw  is  the  amount  of  water  molecules  in hydrate molecule, for methane hydrate, nwa10 6. k is the  absolute  permeability  coefficient  of  porous medium,  m2.  Ecker  et  al.[19]  regarded  it  as  the function  of  hydrate  saturation.  krwa11krg  is  relative permeability  coefficient  of  water  and  gas, Bondarev  et  al.  regard  them  as  only  function  of water saturation. a12 w, a12 g is the kinetic viscosity of water  and gas,  Paa1s.  Pwa13Pg  is  the  flow  pressure of water and gas, MPa.  According  to  Kamath  et  al.[17-18],  kinetic  equa-tion of heated hydrate dissociation is:  ))s' Peqgdg ?                          a75a8  Where  Kd?  is  dissociation  constant  of  hydrate under  constant-pressure  heated.  As  is  the  total surface area of hydrates in a unit volume, and may be regarded as a function of hydrate saturation. T is the  temperature  of  unit  volume.  Teq  is  the  phase equilibrium  temperature  of  hydrate  under  the average  pore  pressure  P.  By  using  Dickens  et al.[20]  data,  Brown  (1996)  made  a  more  precise fitting,  and  gained  the  temperature  and  pressure equilibrium  function  of  brine-methane-hydrate system, which is  2105104 )(log1064.8log1009.41083.31 PeqT?+?=a76a8  Then  state  equations,  assistant  equations  are combined with the aforementioned basic equations (1)-  (4),  and  initial  condition  and  boundary condition  are  given,  the  basic  equations  can  be solved by  numerical  method. If  it  is  simplified as radial displacement process, then the basic formula (1)-(4) can be rewritten in polar coordinate, and the position relationship  between the  invasion frontier of  mud  filtrate  and  the  movement  frontier  of hydrate  dissociation  can  be  discussed  and understood  as  well  as  the  change  rules  of  pore pressure  and  water-gas-hydrate  saturation  during the  invasion process.  Here  we  just  discussed how to  build  the  corresponding  model  and  didn?t  give numerical resolution and examples about the model.  CONCLUSION AND SUGGESTION Drilling  fluids  will  inevitably  invade  into  the hydrate-bearing  formation  and take  place  heat  and mass  transfer  action under  the  drive  of  differential pressures  and  temperature,  which  will  cause displace  and  hydrate  surrounding  borehole  to  be decomposed.  The  invasion  process  is  actually coupled  with  heat  conduction  and  the  hydrate dissociation,  and  the  interactions  between  drilling fluids and formation influence the mechanics, pore water  pressure,  capillary  pressure,  water-gas-hydrate saturation, permeability, wave velocity and resistivity  of  the  formation  surrounding  borehole. Therefore,  in  order  to  guide  the  actual  drilling operation  in  the  future,  the  characteristics  of  the drilling  fluids  invading  into  the  hydrate-bearing formation  and  its  influence  rule  should  be thoroughly understood when analyzing on wellbore stability,  well  logging  response  and  formation damage evaluation of hydrate-bearing formation. It can  be  realized  by  establishing  numerical calculation  model  of  fluid  invasion  coupled  with hydrate  dissociation.  On  the  assumption  that hydrate  is  a  portion  of  pore  fluids  and  its dissociation is  a  continuous  water  and  gas  source with  no  uniform  strength,  it  can  describe  the dynamic  process  of  drilling  fluids  invasion  by coupling  Kamath?s  kinetic  equation  of  heated hydrate  dissociation  or  Kim?s  kinetic  equation  of depressurization  dissociation  into  mass conservation equations.  But  there  also exists  some problems  such  as  hard  to  quantify  relationship between  hydrate  saturation  and  permeability,  the distribution  pattern of  hydrate  in sediment  and its surface area in porous medium. Besides, the kinetic equations  of  hydrate  dissociation  themselves  are not  perfect.  All  these  require  studying  thoroughly and discussing deeply in the future.  REFERENCES [1] Klauda J.B., Sandler S. I. Global distribution of methane  hydrate  in  ocean  sediment.  Energy  & Fuels 2005; 19(2): 459-470 [2] Ning F.L. 2005. Study on the Wellbore Stability in  Gas  Hydrate  Formation.  Doctoral  dissertation, WuHan: China University of GeoSciences,2005 [3]  Collett  T.S..  Energy  resource  potential  of natural  gas  hydrates.  AAPG  Bulletin  2002;  86: 1971-1992 [4]  Maurer  W.  Gas  Hydrate  Drilling  Problems. Gulf  of  Mexico.  Hydrates  R&D  Workshop Proceedings, 2000.p40-57 [5]  Sheila,  N.,  Richard,  B.,  Pat,  H.  Wellbore Stability  Model(WBS)  for  Sediments  Containing Gas  Hydrates,  Gulf  of  Mexico  DOE-JIP.  JIP Workshop, Denver, 2003 [6] Tan C.P., Freij-Ayoub R., Clennell M.B., et al. Managing  Wellbore  instability  Risk  in  Gas-Hydrate-Bearing  Sediments.  SPE  92960.  Asia Pacific  Oil&Gas  Conference  and  Exhibition, Jakarta, Indonesia, 2005.  [7] Freij-Ayoub R., Tan C.P., Clennell M.B., et al. A  Wellbore  Stability  Model  for  Hydrate  Bearing Sediments.  Journal  of  Petroleum  Science  and Engineering 2006; 57:209-220. [8]  Sun T.,  Cheng  L.Y.,  Qiu C.J.,  Zhu  Z.P.  Study on  Performance  of  Low  Temperature  Drilling Fluids  Used for  Gas  Hydrate  Exploration.  Natural Gas Industry (China) 2004; 24(2):61-63 [9] Winters,W.J., Dallimore,S.R., Collett,T.S., Kat-sube T.J., Jenner K.A., Cranston R.E., Wright J.F., Nixon F.M.,  and Uchida  T.  Physical  properties  of sediments  from  the  JAPEX/JNOC/GSC  Mallik2L-38 gas hydrate research well. Geological Survey of Canada Bulletin 1999; 544: 95-100 [10]  Winters  W.J.,  Pecher  I.A.,  Booth J.S.,  Mason D.H.,  Relle  M.K.,  and  Dillon  W.P.  Properties  of samples  containing  natural  gas  hydrate  from  the JAPEX/JNOC/GSC  Mallik2L-38  gas  hydrate research well,  determined  using  Gas  Hydrate  and Sediment  Test  Laboratory  Instrument  (GHASTL I).Geological Survey of Canada Bulletin 1999; 544: 241-250 [11]  Winters  W.J.,  Pecher  I.A.,  Waite  W.F.,  and Mason  D.H.  Physical  properties  and rock  physics models  of  sediment  containing  natural  and laboratory-formed methane gas hydrate. American Mineralogist 2004; 89: 1221-1227 [12]  Xu  W.Y.  Modeling  dynamic  marine  gas hydrate  systems.  American  Mineralogist  2004;  89: 1271-1279 [13]  Holder  G.  D.,  Angert  P.  F.,  Godbole  S.  P. Simulation  of  Gas  Production  from  a  Reservoir Containing  Both  Gas  Hydrates  and  Free  Natural Gas.  Proceedings  of  57th  Society  of  Petroleum Engineers  Technology  Conference.  SPE Paper11005, 1982, P26-29 [14]  Makogon  Y.F.,1997.  Hydrates  of  Hydro- carbons.  Tulsa:  PennWell  Publshing  Co.,  1997. p.482 [15]  Goel  M.W.,  Subhash  Shah. Analytical  Mode-ling  of  Gas  Recovery  from  in  Situ  Hydrate Dissociation. Petro. Sci. Eng. 2001; 29(2): 115-127  [16]  Yousif  M.H.,  Sloan  E.D.  Experimental  and Theoretical  Investigation  of  Methane?Gas?Hydrate  Dissociation  in  Porous  Media.  SPE Reservoir Eng. 1991; 6(4): 452-458 [17]  Kamath  V.A.,  Holder  G.D.,  Angert  P.  F. Three  Phase  Interfacial  Heat  Transfer  During  the Dissociation of  Propane  Hydrates  .Chem.Eng.Sci. 1984; 39(10): 1435? 1442 [18] Kamath V.A.,  Holder G.D.  Dissociation Heat Transfer  Characteristics  Methane  Hydrates. AIChE J. 1987; 33(2): 347-350 [19]  Ecker  C.,  Dvorkin J.,  Nur  A.  Sediments  with gas hydrates: Internal structure from seismic AVO. SEP 1995; 89: 1-24 [20]  Dickens  G.R.,  Quinby,  Hunt  M.S.  Methane hydrate stability in seawater. Geophysics Research Letter 1994; 21(19): 2115-2118 

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